Certain types of machines, such as an excavator, for example, include a swing mechanism which enables an upper structure to be rotated about a base machine on a central pivot by a hydraulic swing motor. The hydraulic swing motor is part of a hydraulic circuit that includes a directional control valve configured to control the swing motor. The large mass and geometry of the upper structure of the machine create high inertial loads when the upper structure is rotated.
Many devices have been employed in the hydraulic circuit of such machines to prevent or reduce the inertia-induced hydraulic shock loads on the various parts of the machine and the hydraulic circuit. One such example is disclosed in U.S. Pat. No. 4,586,332, which issued on May 6, 1986, to Lawrence F. Schexnayder. The hydraulic swing motor control circuit described in the '332 patent includes a pair of shunt valves each of which establishes restricted communication between first and second motor conduits leading to the hydraulic swing motor in a particular direction at their normal spring-biased position. This allows limited free swing of the upper structure when the directional control valve is shifted from an operating position to the neutral position. Shifting the directional control valve to an operating position causes an appropriate one of the shunt valves to shift to a blocking position so that no interconnection between the motor conduits exists. The present disclosure is directed to improving machine productivity and fuel efficiency through the swing motor operation.